Characterizing and troubleshooting wide-band RF systems

The proliferation of wireless devices and an unrelenting demand for higher data rates have placed a significant strain on the RF spectrum. As bandwidth requirements for RF components and systems continue to increase, spectrum availability has become a serious challenge. System operators must use every hertz of the RF spectrum as efficiently as possible, yet they must also take great care to avoid interference with other signals that are closer and more prevalent than ever before. Of course, all of this must be accomplished as quickly and with the lowest capital expenditure possible, resulting in a classic engineering dilemma.

These divergent requirements are driving considerable innovations in RF communications. Advances in digital signal processing combined with strides in analog-to-digital (ADC) and digital-to-analog (DAC) technologies have enabled new generations of remarkable networks and systems. Yesterday's narrow band, single-carrier, triple conversion systems are being replaced with wide band, multi-carrier transmitters enabled by digital signal processing (DSP) and ADCs that produce direct IF, or even direct RF, outputs to the RF amplifier. And waveforms are now digitally pre-distorted for maximum efficiency and tight spectrum control.

Troubleshooting an RF design now requires the ability to trace a signal from a DSP-generated baseband to a wide band digitally modulated RF output. These digitally generated RF signals create new, transient faults that previous generations of RF test equipment are unable to discover, trigger on and measure. In addition, optimizing wide band systems that use digital pre-distortion (DPD) in the transmit chain requires the creation of a pre-distorted waveform.

Fortunately, advanced real-time spectrum analyzers (RTSAs) are available to facilitate the acquisition, measurement, and analysis of digitally-modulated and pre-distorted RF signals. With wide capture bandwidth, deep memory, and inherently correlated measurements, these instruments enable the efficient and accurate characterization and troubleshooting of today's wide-band RF systems.

Digitally Modulated Signals
When an RTSA is used to measure the vector parameters of modulated signals, the instrument acts as the receiver in a Transmit/Receive (Tx/Rx) pair. Figure 1 illustrates the components of a generic Tx/Rx chain and the role the RTSA plays in replacing the Rx function. The receive chain begins with a low-noise RF amplifier tuned to the receive frequency. (In an RTSA, a pre-amplifier may be used for low level signal measurements, but is not needed for high level measurements of transmitters.)

Like a receiver, the RTSA contains an intermediate frequency (IF) filter for spurious and interfering signal control. Its bandwidth is that of the instrument's capture bandwidth, which may allow unwanted signals into the measurement.

1. The RTSA acts as the receiver in a Tx/Rx pair when measuring vector signal parameters.

Vector measurements of digitally modulated signals require the incoming signal to be compared to an ideal signal of the same modulation type and data. The signal analyzer must be aware of, and capable of reproducing, the modulation parameters of the signal, including frequency, symbol rate, modulation type, transmit/receive filters, and transmitted symbol values.

Once the signal has been demodulated and the reference signal constructed, vector measurements can be performed, such as error vector magnitude (EVM), magnitude error, phase error, origin offset, gain imbalance, and rho (Figure 2).

2. Examples of vector measurements made by a RTSA or VSA. Other panels display magnitude vs. time, EVM vs. time, and constellation display of the same time period.

PAR and CCDF
Modern amplifiers use sophisticated techniques to limit the peak-to-average-ratio (PAR) of the amplified signal in order to optimize output distortions and amplifier efficiency.

PAR is the ratio of a signal's peak power compared to its average power over a defined period of time. Complementary cumulative distribution function (CCDF) is a statistical characterization that plots power level on the x-axis and probability on the y-axis of a graph. Each point on the CCDF curve shows what percentage of time a signal spends at or above a given power level. The power level is expressed in dB relative to the average signal power level.